Typical integrated circuits contain an array of transistors which are covered by layers of dielectric through which conductive lines run. Illustratively, these conductive lines may be made from aluminum with conductive underlayers and conductive overlayers of other materials. Typical fabrication processes involve the blanket deposition of aluminum layers, i.e. by sputtering. (As just mentioned, other conductive layers may be deposited prior to the aluminum layer and subsequent to the aluminum layer.) The aluminum layer, together with any underlayers and overlayers, is patterned by lithographic methods known to those of skill in the art to create conductive lines.
Each integrated circuit manufacturer has one or more processes or recipes for the formation of metal runners, such as aluminum, with its associated underlayers and overlayers. Each recipe may include sputtering times, metal composition, temperatures, thicknesses, etc.
Those concerned with the development of integrated circuits have, however, noticed that apparently satisfactorily manufactured integrated circuits may often fail in subsequent use. A variety of phenomena are associated with these failures. One well documented failure mechanism is termed "stress migration voiding". Stress migration voiding affects the long term reliability of the integrated circuits. Another factor affecting long term reliability is extrusion of the aluminum runner material. Since the aluminum runner undergoes more compression at the bottom of the runner then at the top, a stress gradient exists and the runner may be extruded near its bottom. Another problem which affects long term reliability is decohesion of the aluminum. Aluminum does not always adhere well to underlying dielectrics such as silicon dioxide. Sometimes underlayers such as titanium or titanium nitride do not assist in adhesion of the aluminum because the titanium oxidizes, thereby destroying its adhesive properties. (Some integrated circuits utilize aluminum fuses which do not have underlayers--decohesion is a particularly worrisome problem for these circuits.)
Long term reliability issues are particularly worrisome to integrated circuit manufacturing process designers because, of course, these reliability failures do not occur until the already-manufactured devices are in field use. Thus, a manufacturing engineer may be confronted with the unhappy prospect of being forced to modify his production process after hundreds or thousands of integrated circuits have been manufactured because those circuits in field use have exhibited long term reliability problems.
Thus, there exists a need to predict where a metal conductor, such as an aluminum runner will fail after leaving the factory; to predict when the runner might fail; and then, to determine how, if at all, the manufacturing processes might be modified to prevent such a failure in the future.
Those concerned with fabrication of other types of perhaps macroscopic metallized structures are confronted with similar problems.